Cooling
Cooling Load Reference
Heat source contributors, typical safety margins, redundancy concepts and cooling-failure temperature-rise guidance for technical and server rooms. Use alongside the Cooling Load Calculator.
Last updated: June 2026
Terms
- ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers
- The US industry body that publishes the standard temperature and humidity ranges data-center equipment is designed to handle. When a chart says "ASHRAE recommended range", it means the safe zone most server makers design for.
- PDUPower Distribution Unit
- Essentially a heavy-duty power strip built into a server rack, splitting one incoming power feed into multiple outlets for the equipment inside.
- UPSUninterruptible Power Supply
- A battery backup that keeps equipment running for a short time if mains power cuts out, giving generators time to start or equipment time to shut down safely.
Total Cooling Load — The Building Blocks
Total load = IT load + UPS/power loss + lighting + people + extra heat, then × (1 + safety margin)
IT load is the connected electrical load of servers, network and storage — almost all of it converts to heat. UPS/power-conversion loss adds further heat inside the room (or an adjacent electrical room) on top of the IT load itself.
Heat Source Contributors — Typical Values
Planning-stage values. Always confirm with actual nameplate/measured data before finalising a cooling design.
| Source | Typical value | Notes |
|---|---|---|
| IT equipment load | 1:1 with electrical draw | Essentially all electrical power into IT equipment becomes heat |
| UPS / power-conversion loss | 5 – 10 % | Of IT load; double-conversion UPS typically 6–10 %, line-interactive lower |
| Lighting | 8 – 15 W/m² | LED technical lighting; older fluorescent fixtures run higher |
| People (sensible heat) | 100 – 130 W/person | Seated, light activity — server room/technical staff |
| Envelope gain (walls/roof/solar) | Site-specific | Often folded into "extra heat" or safety margin for small technical rooms |
Rule of thumb: for a typical double-conversion UPS at 8% loss, a 20 m² room with one occupant and 10 W/m² lighting adds roughly 0.25 kW on top of the IT load before any safety margin.
Safety Margin Guidance
| Scenario | Typical margin | Reasoning |
|---|---|---|
| Stable, fully known load | 10 – 15 % | Mature site, no planned growth, good measured data |
| Typical project (default) | 20 % | Standard planning allowance for measurement uncertainty and minor load drift |
| Growth expected (12–24 months) | 25 – 35 % | Room for additional racks/equipment without re-engineering cooling |
| High-density / HPC / GPU rooms | 30 %+ | Load profiles change quickly; thermal headroom protects against throttling |
Cooling Redundancy Concepts
| Concept | Meaning | Typical use |
|---|---|---|
| N | Exactly enough capacity for the load, no spare unit | Non-critical / lab spaces only |
| N+1 | One extra unit beyond the minimum required | Standard for most server/technical rooms |
| 2N | Two fully independent, fully sized cooling paths | Mission-critical facilities, Tier III/IV designs |
| 2N+1 | Two independent paths, each with its own N+1 | Highest-tier critical infrastructure |
Cooling-Failure Temperature-Rise Guidance
When cooling stops, room temperature rises at a rate driven by IT load density and room thermal mass. These are planning-stage rules of thumb, not a substitute for a proper thermal-runaway calculation.
| Room load density | Approx. rise rate | Time to reach ASHRAE A2 limit (35 °C) from 22 °C |
|---|---|---|
| Low (< 1 kW/m²) | 1 – 2 °C/min | ~7 – 13 min |
| Medium (1 – 3 kW/m²) | 3 – 5 °C/min | ~3 – 4 min |
| High (> 3 kW/m², dense racks) | 6 °C/min or more | < 2 min |
Lower-density technical rooms with significant thermal mass (large room volume relative to load) tolerate cooling outages far longer than dense rack rows with minimal free air volume.
Frequently Asked Questions
Does all IT electrical load really become heat?
Yes, for practical planning purposes. Almost no energy leaves a server room as anything other than heat — there's no significant mechanical work being done. The electrical load measured at the PDU or panel is a very close proxy for the sensible heat load that cooling must remove.
Why does UPS loss matter if the UPS isn't in the server room?
If the UPS sits in the same room (common in smaller technical rooms), its conversion losses add directly to the room's heat load. If the UPS is in a separate electrical room, its losses only affect that room's own cooling — exclude it from the server room calculation but don't forget to size cooling for the UPS room itself.
How is this different from the Airflow CFM Calculator?
This page and the Cooling Load Calculator answer "how many kW/BTU/tons of cooling capacity do I need?" The Airflow CFM Calculator answers "how much air volume (CFM/m³h) does that cooling capacity require, at a given temperature rise?" Use cooling load first, then airflow.
What if my room has no people or minimal lighting?
Set those inputs to zero or near-zero — unstaffed, motion-sensor-lit technical rooms genuinely have negligible people/lighting load compared to IT load. The safety margin already covers small unmodelled contributors.